This application claims the priority of Korean Patent Application No. 10-2004-0095906, filed on Nov. 22, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to an electrophotographic developing agent. More particularly, the invention relates to an electrophotographic developing agent including at least two types of strontium titanate particulate components having different average primary particle diameters, for use in a developing apparatus of an electrophotographic image processing device.
2. Description of the Related Art
Electrophotographic image processing devices such as laser printers, facsimile machines, copying machines, etc. are now widely used. These devices form a desired image by forming a latent image on a photoreceptor using a laser, moving toner onto the latent image using an electric potential difference, and then transferring the toner image onto a printing medium such as paper.
FIG. 1 illustrates an embodiment of a non-contact developing-type image forming apparatus. Although not described in detail herein, the present invention can also be applied to a contact developing-type image forming apparatus.
In the non-contact developing-type image forming apparatus of FIG. 1, a non-magnetic one-constituent developing agent 8 is fed to a developing roller 5 by a feeding roller 6 including an elastic member, such as a polyurethane foam, sponge, etc. As the developing roller 5 rotates, the developing agent 8 fed to the developing roller 5 arrives at a line of contact between a developing agent regulating blade 7 and the developing roller 5. The developing agent regulating blade 7 includes an elastic member which is made of metal, rubber, or other suitable materials. Only a thin layer of the developing agent 8 can pass between the developing agent regulating blade 7 and the developing roller 5. The thin layer of the developing agent on the roller 5 is electrically charged. The thin layer of developing agent 8 is then rotated to a developing area where it is transferred from the developing roller 5 to an electrostatic latent image formed on a photoreceptor 1.
The developing roller 5 and the photoreceptor 1 face each other and are separated by a predetermined distance. The developing roller 5 rotates counterclockwise and the photoreceptor 1 rotates clockwise. The developing agent 8 is applied to the developing area and is transferred onto the electrostatic latent image of the photoreceptor 1 by force generated by an electric potential difference between the developing roller 5, to which a DC-offset AC voltage is applied. The latent image formed on the photoreceptor 1.
As the photoreceptor 1 rotates, the developing agent 8 on the photoreceptor 1 arrives at a transfer means 9 and is transferred onto a printing paper 13 by the transfer means 9 to form an image. Here, the transfer means 9 may use a corona discharge or may have a roller form. The transfer means 9 is maintained at a high voltage of opposite polarity to the developing agent 8.
The image transferred to the printing paper 13 is fused onto the printing paper 13 by passing through a high-temperature and high-pressure fusing apparatus (not shown). Meanwhile, residual developing agent on the developing roller 5 is recovered by a feeding roller 6 which contacts the developing roller 5. This process is repeated.
In a non-magnetic one-constituent non-contact developing apparatus, a developing agent obtains electric charge through frictional contact with a developing agent carrier, a developing agent regulating blade, and a feeding member. And since a latent image carrier and the developing agent carrier are separated by a predetermined distance, the developing agent is transferred by electrical force alone. Thus, when the developing agent is charged below the proper level, an insufficient amount may be transferred to the latent image, thereby reducing image density. And, when the developing agent is charged above the proper level, it may be transferred to a non-image area, thereby causing image contamination.
Moreover, even if the developing agent initially shows proper charge characteristics, as a printing process is repeated, external additives of the developing agent either become embedded in a resin of the developing agent due to stress caused by friction between the developing agent and the developing agent regulating blade, or are separated from the developing agent. If this occurs, the mobility of the developing agent decreases, and physical absorbency between the developing agent carrier, the developing agent regulating blade, and the developing agent increases. Thus, uniform frictional charging of the developing agent may not occur such that the developing agent is not charged to the proper level or is charged with the wrong polarity, thereby reducing image density or causing image contamination.
Increasing the amount of external additives to prevent such problems causes an increase in the amount of frictional charge of the developing agent and hence an increase in the force between the developing agent and the developing agent carrier. However, increasing the amount of the external additives decreases the image density. Also, as the amount of external additives is increased, the ability of a cleaning blade contacting the latent image carrier to effectively remove all remaining developing agent deteriorates causing a charging roller to be contaminated. In addition, the residual developing agent or impurities remain on the latent image carrier. This deteriorates image quality by causing spots and vertical white/black lines in an image.
Japanese Patent Publication No. 1999-212293 discloses a non-magnetic one-constituent developing agent containing at least one type of inorganic material and strontium titanate. Japanese Patent Publication No. 2003-202702 discloses a negatively chargeable toner containing two types of silicas with different mean particle diameters as external additives, and hydrophobic titanium oxide. However, these developing agents cannot obtain a uniform image density when printing for a long time and produce poor quality, blurry images.